Spring winding devices for forming coil springs have been described previously. U.S. Pat. No. 4,893,491 to Ohdai et al., issued Jan. 16, 1990 and assigned to Asahi-Seiki Manufacturing Co., Ltd., describes a spring winding machine and a method for forming coil springs. A wire is fed from a quill against a forming surface. The forming surface is moved to a forward position close to a tip of the quill by a drive system including a cam assembly under numerical control. As the wire is fed from the quill against the forming surface, the wire is bent. By rotating the forming surface, the wire fed from the quill can be bent in different directions such that coil springs of different predetermined forms and sizes are formed. After the coil spring reaches its predetermined form and size, the forming surface is withdrawn from the tip of the quill by the drive system and cam assembly and a cutting tool is moved to a forward position to cut the formed coiled spring away from the wire fed from the quill.
The spring winding device described by Ohdai et al. in the '491 patent provides various cutting and bending tools mounted on a tool mounting frame. These tools can be repositioned relative to the tip of the quill in the same manner as the forming surface. The tool mounting frame includes an opening through which the quill can pass. This allows tools to be mounted and brought in towards the quill and withdrawn from the quill from both front and rear sides of the tool mounting frame. By providing a mechanism for mounting various tools in close proximity to the tip of the quill, a wide variety of bends can be made when forming coil springs. Furthermore, by utilizing numerical control of the apparatus, coil springs can be formed which have a variety of dimensions.
However, because of variations in the dimensions of the wire, variations in the stiffness of spring winding machines, and variations in the wear of the tools on the machines, it is very difficult to form a spring or a batch of springs for certain uses where tight tolerances are required. This is at least partially because the forming surface of the spring winding machine can only be repositioned relative to the tip of the quill with limited precision.
A roll or lot of wire fed from the quill, whether having a round, rectangular, barrel, or any other cross-sectional structure includes many variations. Variations exists in the chemistry depending upon the amount of carbon which is included in the chemical composition of the wire. The different chemical compositions will vary the tensile strength of the wire and also the modulus of elasticity. These wire variations lead to the difficulty of winding springs with tight tolerances.
Differences in lubrication on the wire will change the amount of force which must be applied to the wire at the forming surface in order to bend the wire because of the different surface condition and surface finish. These differences in surface conditions and surface finishes lead to variations in dimensions of coil springs as they are being formed at the forming surface. In addition, because the wire is metal, variations in temperature will contract and expand the wire to a certain degree, leading to further variations in the dimensions of the coil springs.
Many stresses are also incorporated into a lot or a roll of wire used for forming springs. Not only does the tensile strength vary in such a lot of wire, because of the manner in which it was drawn, but the drawing process puts different cast and helix into the wire. Furthermore, the wire is shipped in a large coil. Therefore, the wire is going to have a different cast and helix throughout the coil, from the interior to the exterior.
In addition, many springs are formed by the same tools and the same forming surfaces of a certain spring winding machine. Due to the repetitious forces applied to the tools, the surfaces of the tools will vary over time in response to wear. This wear results in additional inconsistencies when forming springs.
Because of the variations in the condition or other parameters of the wire, the wear on the forming surface and other tools used to bend the wire, and the variations in the positioning of the forming surface as it is repositioned proximate the tip of the quill, significant inconsistencies in various parameters of individual coil springs within any particular batch of coil springs will occur. In many industrial applications it is necessary to provide coil springs having consistent parameters with narrow tolerances. These tolerances must be narrow both in regard to individual wraps within a single spring and in regard to individual springs within a batch or lot of springs. Applications for springs requiring such narrow tolerances include use within wrap spring clutches for office copy machines, such as those manufactured by Reell Precision Manufacturing Corporation, St. Paul, Minn.
Various other aspects of spring winding can lead to further inconsistencies in the parameters of a single spring and/or a batch of springs. For example, when a spring is cut after being formed by a spring winding machine, as described above, if the spring is allowed to drop a distance to a surface, stress is added and a widening of the distribution of various parameters occurs. In addition, inconsistent heat treating of the springs after they are bent by the forming surface can lead to additional variations in parameters of the coil springs. Therefore, if springs are positioned in an oven where springs are heat treated at a temperature which does not have a uniform profile, variations in the dimensions or other parameters of the spring can occur.